In plastic encapsulated integrated circuit (IC) packages the die is wire bonded, or connected by a tape automated bonding (TAB) process, to a metal leadframe which connects the die input/output (I/O) bonding pads to external electrical circuitry. The leadframe includes a plurality of spaced apart electrical conductors, or leads arrayed in a pattern. The leads are typically copper or iron-nickel alloy. The lead outer ends are formed integrally with a frame tie-bar at the time of manufacture, and they radiate inwardly in a central direction. The inner ends of the leads are more or less in register with, or at least in proximity to, associated ones of the die bonding pads. The gap, or spacing between leads at the outer end is limited by the dimensions of the package, while the inner end gap is defined by the die bonding pads.
The increasing density of the IC dies (gates per square inch) requires a corresponding decrease in the spacing of I/O pads making the required inner lead spacing increasingly smaller. Typically the leadframe is produced by stamping the pattern from blank metal foil or by etching, using photolithographic processes. As inner lead spacing becomes smaller both methods are limited in their ability to provide a minimum gap dimension.
Stamping produces a minimum gap value approximately equal to the thickness of the base material, due to displacement of the stamped material. In addition, with finer frame patterns stamping may induce strains in the leads which can cause lead fracture. This, in turn, causes IC failure. Finer lead spacing also requires higher precision stamping tools, so that tool wear and breakage become more serious.
Etching does not produce the lead strain that occurs with stamping, however, minimum gap dimension is also dependent on the base material thickness. A chemical etchant dissolves material laterally as well as normal to the surface, so that single side etching of a base material of thichness T produces a minimum aperture diameter of 2T. For a 0.008 inch (0.203 mm) thick copper leadframe, single side etching produces a minimum gap of 0.016 inches (0.406 mm).
U.S. Pat. No. 4,389,279 to Weber et al. discloses a method of etching a metallic strip simultaneously from both sides to produce a color television shadow mask, whereby the mask apertures can be controlled to a minimum diameter equal to the strip thickness. Therefore, simultaneous etching of both sides to a depth of T/2 produces a minimum aperture of T. As a result etching, as with stamping, limits an etched aperture minimum diameter to a value approximately equal to the base material thickness. Since frame thickness determines frame mechanical strength, there is a trade-off between strength and gap spacing requirements. A leadframe with a material thickness of 0.008 inch (0.203 mm) for sufficient strength, is limited to inner lead end gaps of 0.008 inches, which may be too great for very large scale integrated (VLSI) IC's.